JP3447120B2 - MRI equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to MR (Magnetic R).
esonance) imaging method and MRI (Magnetic R
esonance Imaging) device. More specifically, it relates to an MR imaging method and an MRI apparatus for applying a binomial pulse in order to selectively saturate bound water protons.

[0002]

2. Description of the Related Art At least two amplitudes are alternately inverted.
The binomial pulse that consists of two parts and the ratio of the absolute value of the time-amplitude product of each part becomes a binomial coefficient, and the sum of the time-amplitude product of each part becomes 0 constitutes a body fluid with a small molecular weight. It is applied before the data acquisition pulse sequence in order to selectively excite and saturate the bound water protons that constitute a cell membrane having a large molecular weight without exciting free water protons.

FIG. 5 shows a conventional MR applying a binomial pulse.
It is an example of a pulse sequence of an imaging method. In this pulse sequence S ′, the NMR (Nuclear
Before collecting the Magnetic Resonance) signal, a binomial pulse R′α having a transmission frequency fs equal to the resonance frequency fo of free water protons is applied.

As shown in FIG. 6, the binomial pulse R'α is
Is a 1.2.1 pulse obtained by dividing an excitation pulse having a flip angle .alpha. Of 1: -2: 1, and has a portion B1 of amplitude C and time width .tau., And a portion B2 of amplitude -C and time width 2.tau. , A portion B3 of amplitude C and time width τ, and three portions.

FIG. 7 shows N of free water protons in the human body.
MR spectrum Sf, NMR spectrum Sb of bound water protons, and frequency spectrum S of the binomial pulse R′α.
r ′ is shown. Since the transmission frequency fs of the binomial pulse R'α is matched with the resonance frequency fo of the free water proton, the peak of the NMR spectrum Sf of the free water proton coincides with the valley of the frequency spectrum Sr 'of the binomial pulse R'α. . Since the NMR spectrum Sf of free water protons has a relatively narrow band, most of free water protons are not excited by the binomial pulse R′α. On the other hand, since the NMR spectrum Sb of the bound water protons has a relatively wide band, it is excited by the binomial pulse R′α.

[0006]

In the conventional pulse sequence of the MR imaging method for applying the binomial pulse, the binomial pulse R'at the resonance frequency fo of the free water protons.
By adjusting the transmission frequency fs of α, the peak of the NMR spectrum Sf of free water protons is the binomial pulse R′α.
Of the frequency spectrum Sr ′ of FIG. However, as can be seen from FIG. 7, the NMR spectrum Sf of free water protons in the human body is asymmetric with respect to the peak due to the influence of components such as fat. That is, it is biased toward the lower frequency side. Therefore, as shown in FIG. 8, there is a problem that the free water protons corresponding to the portion A on the low frequency side are excited by the binomial pulse R′α, and the signal intensity of the free water protons decreases. is there. Therefore, an object of the present invention is to provide an MR imaging method and an MRI apparatus capable of solving the problem that a part of free water protons are excited by the binomial pulse R′α and the signal intensity is reduced. It is in.

[0007]

According to a first aspect of the present invention, the present invention provides an MR imaging method of applying a binomial pulse to selectively saturate bound water protons. Provided is an MR imaging method characterized by lowering the resonance frequency of protons.

In a second aspect, the present invention applies a binomial pulse having a transmission frequency lower than the resonance frequency of free water protons in an MRI apparatus that applies a binomial pulse in order to selectively saturate bound water protons. There is provided an MRI apparatus characterized by comprising pulse applying means.

According to a third aspect of the present invention, in the MRI apparatus having the above structure, the transmission frequency of the binomial pulse is 100 Hz to 20 Hz higher than the resonance frequency (fo) of free water protons.
Provided is an MRI device characterized by being low in the range of 0 Hz.

[0010]

In the MR imaging method according to the first aspect and the MRI apparatus according to the second aspect, the transmission frequency of the binomial pulse is set lower than the resonance frequency of free water protons. As mentioned above, NM of free water protons in the human body
Since the R spectrum is biased toward the frequency lower than the resonance frequency, if the transmission frequency of the binomial pulse is made lower than the resonance frequency of the free water protons accordingly, the free water protons that are excited by the binomial pulse are excited. The part of is reduced. Therefore, it is possible to prevent a decrease in signal intensity of free water protons.

In the MRI apparatus according to the third aspect, the transmission frequency of the binomial pulse is lower than the resonance frequency of free water protons within the range of 100 Hz to 200 Hz. According to the knowledge of the inventor of the present invention, when the transmission frequency of the binomial pulse is lower than the resonance frequency of the free water proton within the above range, the proportion of the free water proton excited by the binomial pulse is usually the smallest. Become.

[0012]

The present invention will be described in more detail with reference to the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 shows an MR according to an embodiment of the present invention.
3 is a block diagram of the I device 100. FIG. This MRI apparatus 10
At 0, the magnet assembly 1 has a space portion (hole) for inserting the subject therein, and a main magnetic field coil that applies a constant main magnetic field to the subject so as to surround this space portion, Gradient magnetic field coil for generating a gradient magnetic field (the gradient magnetic field coil includes coils for each of X, Y, and Z axes), and a transmission coil for applying an RF pulse for exciting protons in the subject. And N from the subject
A receiving coil and the like for detecting MR signals are arranged. The main magnetic field coil, the gradient magnetic field coil, the transmitting coil and the receiving coil are respectively the main magnetic field power source 2, the gradient magnetic field driving circuit 3,
It is connected to the RF power amplifier 4 and the preamplifier 5.

The sequence storage circuit 8 operates the gradient magnetic field driving circuit 3 based on a sequence such as the spin-warp method according to a command from the computer 7 to generate a gradient magnetic field from the gradient magnetic field coil of the magnet assembly 1. At the same time, the gate modulation circuit 9 is operated to modulate the high frequency output signal from the RF oscillating circuit 10 into a pulse-shaped signal having a predetermined timing and a predetermined envelope, which is added as an RF pulse to the RF power amplifier 4 and the RF power amplifier 4 After the power is amplified by, the power is applied to the transmission coil of the magnet assembly 1 to selectively excite a target excitation region.

The preamplifier 5 includes a magnet assembly 1
The NMR signal from the subject detected by the receiving coil is amplified and input to the phase detector 12. The phase detector 12 is
The output of the RF oscillation circuit 10 is used as a reference signal, and the preamplifier 5
The NMR signal from is phase-detected and given to the A / D converter 11. The A / D converter 11 converts the analog signal after phase detection into a digital signal and inputs it to the computer 7.
The computer 7 performs an image reconstruction operation on the digital signal from the A / D converter 11 to generate an image (proton density image) of the target excitation region. This image is displayed on the display device 6. In addition, the computer 7 is a console 13
Responsible for overall control such as receiving information input from. The MR imaging method of the present invention is executed by the computer 7, the sequence storage circuit 8, the gate modulation circuit 9, the RF power amplifier 4, and the transmission coil of the magnet assembly 1. That is, these correspond to pulse applying means.

FIG. 2 shows an embodiment of the pulse sequence of the MR imaging method for applying the binomial pulse according to the present invention. In this pulse sequence S, the NM from free water protons is generated by the data acquisition pulse sequence D.
Before collecting the R signal, the resonance frequency f of free water protons
Transmission frequency fs (= fo-Δf) lower than o by Δf
Is applied. This binomial pulse Rα is called an off resonance binomial pulse. As the transmission frequency fs, as shown in FIG. 3, an intermediate frequency (fa + fb) between the frequencies fa and fb that gives 20% of the peak of the NMR spectrum Sh of the subject (person).
You may set / 2. At this time, Δf = fo− (fa
+ Fb) / 2. Also, Δf is usually 100 Hz
Within the range of ~ 200 Hz. The binomial pulse Rα is
For example, it is 1.2.1 pulse or 1 / 33.1 pulse.

FIG. 4 shows N of free water protons in the human body.
An MR spectrum Sf, an NMR spectrum Sb of bound water protons, and a frequency spectrum Sr of the binomial pulse Rα are shown. The transmission frequency fs of the binomial pulse Rα is made lower than the resonance frequency fo of the free water proton so that the portions Aa and Ab excited by the binomial pulse Rα of the free water protons are as small as possible. Since the NMR spectrum Sf of free water protons has a relatively narrow band,
Free water protons are hardly excited by the binomial pulse Rα. On the other hand, the NMR spectrum S of bound water protons
Since b has a relatively wide band, it is excited by the binomial pulse Rα.

[0017]

According to the MR imaging method and the MRI apparatus of the present invention, it is possible to prevent a part of the free water protons from being excited by the binomial pulse to reduce the signal intensity.

[Brief description of drawings]

FIG. 1 is a block diagram of an MRI apparatus according to an embodiment of the present invention.

FIG. 2 is an exemplary diagram of a pulse sequence for performing the MR imaging method of the present invention.

FIG. 3 is an explanatory diagram showing one of methods for determining a transmission frequency of a binomial pulse according to the present invention.

FIG. 4 is an explanatory diagram of a frequency spectrum of a binomial pulse, an NMR spectrum of free water protons of a human body, and an NMR spectrum of bound water protons of the present invention.

FIG. 5 is an exemplary diagram of a pulse sequence for performing a conventional MR imaging method.

FIG. 6 is a view showing an example of 1.2.1 pulse.

FIG. 7 is an explanatory diagram of a frequency spectrum of a conventional binomial pulse, an NMR spectrum of free water protons of a human body, and an NMR spectrum of bound water protons.

FIG. 8 is an explanatory diagram of partial excitation of free water protons by a conventional binomial pulse.

[Explanation of symbols]

100 MRI apparatus 1 magnet assembly 4 RF power amplifier 7 computer 8 sequence controller 9 gate modulation circuit 10 RF oscillation circuit 11 RF power amplifier 13 console Rα binomial pulse fs binomial pulse transmission frequency Sr binomial pulse frequency spectrum Sh human NMR spectrum of Sf NMR of free water protons of human
Spectrum Sb NMR of bound water protons of human
Spectrum

Claims (3)

(57) [Claims] 1. An MRI apparatus for applying a binomial pulse for selectively saturating bound water protons, comprising pulse applying means for applying a binomial pulse having a transmission frequency lower than the resonance frequency of free water protons. An MRI apparatus characterized by: 2. The MRI apparatus according to claim 1,
An MRI apparatus characterized in that the transmission frequency of the binomial pulse is lower than the resonance frequency of free water protons within a range of 100 Hz to 200 Hz. 3. The MRI apparatus according to claim 1 or 2, wherein the binomial pulse is a 1.2.1 pulse obtained by dividing an excitation pulse having a flip angle .alpha. Of 1: -2: 1. An MRI apparatus characterized by: